Direct positive process involving uniform exposure of an imagewise exposed internally sensitive silver halide emulsion to high intensity radiation

ABSTRACT

POSITIVES IMAGES CAN BE PRODUCED IN A SILVER HALIDE PHOTOGRAPHIC ELEMENT BY (1) IMAGEWISE EXPOSING A PHOTOGRAPHIC ELEMENT CONTAINING A SILVER HALIDE EMULSION WHICH CONTAINS INTERNAL IMAGE SILVER HALIDE GRAINS (A) WHICH ARE INTERNALLY SENSITIZED AND (B) HAVE A HOLE TRAPPING AGENT WHICH IS A SPECTRAL SENSITIZING DYE ADSORBED TO THE SURFACE OF THE SILVER HALIDE GRAINS AND WHICH HAS A POLARGRAPHIC OXIDATION POTENTIAL OF LESS THAN PLUS 0.90 V., (2) EXPOSING THE PHOTOGRAPHIC ELEMENT TO A HIGH INTENSITY OVERALL FLASH OF LIGHT, AND (3) DEVELOPING A POSITIVE IMAGE IN THE PHOTOGRAPHIC ELEMENT WITH A SURFACE, SILVER HALIDE DEVELOPER. SILVER HALIDE PHOTOGRAPHIC EMULSIONS FOR PRODUCING SUCH POSITIVE IMAGE WHICH ARE INTERNALLY FOGGED BUT HAVE LITTLE OR NO SURFACE FOR ARE ALSO DISCLOSED.

United States Patent O Int. Cl. G03c 5/24 U.S. CI. 9664 5 Claims ABSTRACT OF THE DISCLOSURE Positive images can be produced in a silver halide photographic element by (1) imagewise exposing a photographic element containing a silver halide emulsion which contains internal image silver halide grains (a) which are internally sensitized and (b) have a hole trapping agent which is a spectral sensitizing dye adsorbed to the surface of the silver halide grains and which has a polarographic oxidation potential of less than plus 0.90 v., (2) exposing the photographic element to a high intensity overall flash of light, and (3) developing a positive image in the photographic element with a surface, silver halide developer. Silver halide photographic emulsions for producing such positive image which are internally fogged but have little or no surface fog are also disclosed.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to methods of producing a positive image using an internally sensitized silver halide photographic emulsion. In one aspect, it relates to employing certain hole trapping agents on an internally sensitive silver halide photographic emulsion for forming positive images. A further aspect relates to methods employing imagewise exposure followed by a high intensity overall flash of the described silver halide.

State of the art Processes are known in the art for making positive images in unfogged silver halide emulsions. Typical processes are described in U.S. Pats. 2,497,875 of Fallesen, issued Feb. 21, 1950; U.S. 2,588,982 of Ives, issued Mar. 11, 1952 and U.S. 2,456,953 of Knott et al., issued Dec. 21, 1948. Generally, the prior processes used internally sensitive silver halide emulsions such as emulsions produced as described in U.S. Pat. 2,592,250 of Davey et al., issued Apr. 8, 1952. There has been a continuing need, however, to provide processes for providing positive images in photographic silver halide materials with improved photographic characteristics such as increased photographic speed and improved image discrimination.

Photosensitive materials such as photosensitive materials for producing direct positive images employing a spectral sensitizing dye adsorbed to the surface of silver halide crystals in the photographic emulsion are known. Such spectral sensitizing dyes are described, for example, in British Pat. 1,186,718, published Apr. 2, 1970; U.S. 3,364,026 of Rees, published Jan. 16, 1968; and U.S. Pat. 3,537,858 of Wise, issued Nov. 3, 1970.

There has, however, been a continuing need to provide improved photographic processes for producing positive "ice images using spectral sensitizing dyes to provide improved photographic speed.

SUMMARY OF THE INVENTION It has been found, according to the invention, that an improved process for providing increased photographic speed for direct positive images can be provided by (1) imagewise exposing a photographic element containing a silver halide photographic emulsion which contains silver halide grains (a) which are internally sensitive and have a hole trapping agent which is a spectral sensitizing dye adsorbed to the surface of the silver halide grains and which dye has an oxidation potential of less than plus 0.90 v. relative to a Ag AgCl reference electrode, (2) exposing the photographic element to a high intensity overall flash of light, and (3) developing a positive image in the photographic element with a surface, silver halide developer. It has been found especially important to employ internally fogged silver halide grains which contain the described spectral sensitizing dyes having an oxidation potential of less than plus 0.90 v. as illustrated in the following comparative examples.

DETAILED DESCRIPTION OF THE INVENTION A range of spectral sensitizing dyes are suitable in the practice of the invention as hole trapping agents. It is important that the spectral sensitizing dyes have an oxidation potential of less than plus 0.90 v. and a reduction potential more negative than -1.00 v. Although the mechanism of the so-called flash reversal described is not clearly understood, it is believed that the improved effects are due to the retention of spectrally sensitized positive holes on the surface of the emulsion grains and the capture of photo electrons at the interior of the silver halide grains. For this reason the term hole trapping agent as employed herein is intended to mean a compound which provides this and is preferably a certain dye having the described oxidation potential and reduction potential. If the spectral sensitizing dye has an oxidation potential greater than plus 0.90 v., it is believed that spectrally sensitized bleaching of internal fog is possible; but if the oxidation potential of the dye is less than plus 0.90 v., it is believed that no spectrally sensitive bleaching of internal fog occurs at a pAg of 8.2 which indicates the spectrally sensitized positive holes are restricted to the surface and interfere strongly with latent image formation that may occur at the surface during the described process. Results of processing, according to the invention, in many cases, give the appearance of a solarized image; however, the severity of solarization like response mainly in the spectrally sensitized region and not as much in the region of absorption of the silver halide is believed surprising.

Spectral sensitizing dyes which are suitable having the described oxidation potentials include dye known to be useful in spectral sensitization of silver halide photographic emulsions and are typically characterized as methine or polymethine dyes having oxidation potentials of less than plus 0.90 v. and reduction potentials more negative than minus 1.00 v. Typical useful classes of dyes which can be used in the process of the invention include methine dyes such as cyanines, isocyanines, pseudocyanines, hemicyanines, merocyanines, oxonols, azacyanines and the like. The dye useful according to the invention are characterized as being spectral sensitizing dyes for silver halide emulsions and have a radiation adsorption peak in the ultraviolet and/ or visible and/ or infrared regions of the electro-magnetic spectrum.

The oxidation potentials of a dye useful in the process of the invention can be measured employing procedures known in the art.

The electrochemical potential measurements can be made with an approximately 10* molar solution of the dye in an electrolyte; for example, a methanol which is 0.05 molar in lithium chloride. A dropping mercury electrode can be used for the cathodic measurement with the polarographic halfwave potential for the cathodic response most positive in potential designated B A pyrolytic graphite electrode can be used for the anodic measurement with the voltammetric half-peak potential for the anodic response most negative in potential designated E,,. In each measurement, the reference electrode can be an aqueous silver-silver chloride (saturated potassium chloride) electrode at 20 C. Plus and minus signs are assigned to the potential values according to the IUPAC Stockholm Convention 1953. The E, and E values so measured shall not include processes in which electron transfer is primarily the result of the presence in solution of the counter ion of a positively charged dye or other such chemical entities in solution that are not an integral part of, or attached to, the chromophoric system of the dye. A response of lesser current magnitude preceding the primary response, such as a prewave resulting from adsorption of the electrolysis product to the electrode surface, shall be excluded from designation as E,, or E Electrochemical measurements of this type are known in the art and are described in one or more of the following reference texts: New Instrumental Methods in Electrochemistry, by Delahay, Interscience Publishers, New York, N.Y., 1954; Polarography, by Kolthoif and Lingane, 2nd edition, Interscience Publishers, New York, N.Y., 1952; and Electrochemistry at Solid Electrodes, by Adams, Marcell Dekker, Inc., New York, N.Y., 1969.

Examples of suitable dyes which satisfy the described test are set out in the following examples. Dyes which satisfy the described oxidation potential test are set out also, for example, in US. application Ser. No. 123,005 of Evans, filed Mar. 10, 1971.

Silver halide photographic emulsions which are useful in the practice of the invention include those which contain internally sensitive silver halide grains. The internally sensitive silver halide grains can be formed by a range of methods but are typically formed by the method described in Example 1 following. Emulsions which are especially useful are internally fogged emulsions as described in Example 1. Internal image-forming emulsions, however, can be prepared by conversion techniques, techniques for preparing core-shell emulsions with chemically sensitized cores, emulsions precipitated in the presence of foreign metal ions and the like.

The internally sensitive emulsions useful in the practice of the invention can be characterized as those which when coated on a film support, image exposed, treated for minutes at 20 C. in a potassium ferricyanide solution and developing for 5 minutes at 20 C. in Kodak Developer D19 plus 0.5 gram per liter of potassium iodide will have a relative speed measured at .3 above fog equal to or greater than a similar coated emulsion image exposed and developed for 5 minutes at 20 C. in Kodak Developer D-19.

The preferred internally fogged emulsions useful in the practice of the invention can be characterized as those which when examined according to normal photographic testing techniques, by coating a test portion on a film support, with no image exposure and developing for about 5 minutes at 20 C. in Kodak Developer D -19 (surface developer), have a maximum density (fog) less than 0.25 and, when developing a separate coated sample for about 5 minutes at 20 C. in Kodak Developer D-19 containing 0.5 g. of potassium iodide per liter (internal developer), have a maximum density greater than 0.5.

A sample of the internally sensitive emulsion can be imagewise exposed to a tungsten light source in a wedge spectrograph until solarization is observed when the exposed sample is developed for about 5 minutes at 20 C. in Kodak Developer D-l9.

When a separate sample of this internally sensitive emulsion contains a hole trapping agent, as described, less image exposure is required when processed in a surface developer to obtain a solarized image, e.g., developed for about 5 minutes at 20 C. in Kodak Developer D-19.

Preferred hole trapping agents are those that, when added to the described internally sensitive emulsion, will reduce the image exposure at least 0.6 log E to obtain a solarized image equal to a control internally sensitive emulsion which is imagewise exposed in the absence of the hole trapping agent.

The concentration of hole trapping agent in an internally sensitive silver halide emulsion used in the practice of the invention can vary depending upon the particular silver halide, the developer, desired image and the like. The concentration of hole trapping agent is a sensitizing concentration of usually about 40 milligrams to about 400 milligrams of hole trapping agent per mole of silver halide.

The silver halides used according to the process of the invention are internally sensitive silver halide grains. Such silver halide emulsions contain substantial internal latent image sites, and preferably are silver halide grains which are internally fogged.

The intensity of the high intensity overall flash of light according to the process of the invention is usually at least 10 times the intensity of the original imagewise exposure.

The photographic elements of this invention can contain incorporated developing agents such as hydroquinones, catechols, aminophenols, 3-pyrazolidones, ascorbic acid and its derivatives, reductones and phenylenediamines. Combinations of developing agents can be employed in the practice of the invention. The developing agents can be in a silver halide emulsion and/or in another suitable location in the photographic element. The developing agents may be added from suitable solvents or in the form of dispersions as described in Yackel, US. Pat. 2,592,368, and Dunn et al., French Pat. 1,505,- 778. It is important that the developers not adversely affect the desired direct positive image produced according to the invention.

The photographic and other hardenable layers used in the practice of this invention can be hardened by various organic or inorganic hardeners, alone or in combination, such as the aldehydes, and blocked aldehydes, ketones, carboxylic and carbonic acid derivatives, sulfonate esters, sulfonyl halides and vinyl sulfonyl ethers, active halogen compounds, epoxy compounds, aziridines, active olefins, isocyanates, carbodiimides, mixed-function hardeners and polymeric hardeners such as oxidized polysaccharides like dialdehyde starch and oxyguargum and the like.

The photographic emulsions and elements described in the practice of this invention can contain various colloids alone or in combination as vehicles, binding agents and in various layers. Suitable hydrophilic materials include both naturally occurring substances such as proteins, for example, gelatin, gelatin derivatives, cellulose derivatives, polysaccharides such as dextran, gum arabic and the like; and synthetic polymeric substances such as water-soluble polyvinyl compounds like poly(vinylpyrrolidone), acrylamide polymers and the like.

The described photographic emulsion layers and other layers of a photographic element employed in the practice of this invention can also contain, alone or in combination with hydrophilic, water-permeable colloids, other synthetic polymeric compounds such as dispersed vinyl compounds such as in latex form and particularly those which increase the dimensional stability of the photographic materials. Suitable synthetic polymers include those described, for example, in U.S. Pats. 3,142,568 by Nottorf, issued July 28, 1964; 3,193,386 by White, issued July 6, 1965; 3,062,674 by Houck et al., issued Nov. 6, 1962; 3,220,844 by Houck et al., issued Nov. 30, 1965; 3,287,289 by Ream et al., issued Nov. 22, 1966; and 3,411,911 by Dykstra, issued Nov. 19, 1968; particularly effective are those water-insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates, those which have cross-linking sites which facilitate hardening or curing, those having recurring sulfobetaine units as described in Canadian Pat. 774,054 by Dy-kstra.

The photographic layers and other layers of a photographic element employed and described herein can be coated on a wide variety of supports. Typical supports include cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polystyrene film, po1y(ethylene terephthalate) film, polycarbonate film and related films or resinous materials, as well as glass, paper, metal and the like. Typically, a flexible support is employed, especially a paper support, which can be partially acetylated or coated with baryta and/or an alpha-olefin polymer, particularly a polymer of an alpha-olefin containing 2 to carbon atoms such as polyethylene, polypropylene, ethylenebutene copolymers and the like.

This invention may be used with elements designed for colloid transfer processes such as described in U.S. Pat. 2,716,059 by Yutzy et al.

This invention may be used with elements designed for color photography, for example, elements containing color-forming couplers such as those described in U.S. Pats. 2,376,679 by Frohlich et a1. 2,322,027 by Jelley et al., 2,801,171 by Fierke et al., 2,698,794 by Godowsky, 3,227,554 by Barr et a1. and 3,046,129 by Graham et al.; or elements to be developed in solutions containing colorforming couplers such as those described in U.S. Pats. 2,252,718 by Mannes et al., 2,592,243 by Carroll et a1. and 2,950,970 by Schwan et al.; and in false-sensitized color materials such as those described in U.S. Pat. 2,763,- 549 by Hanson.

It is understood that the term surface developer as employed herein encompasses those developers which will reveal the surface latent image on a silver halide grain but will not reveal substantial internal latent image in an internal image forming silver halide emulsion and conditions generally used to develop a surface sensitive silver halide emulsion. The surface developers can generally utilize any of the silver halide developing agents or reducing agents but the developing bath or composition is generally substantially free of a silver halide solvent such as water soluble thiocyanates, thioethers, t-hiosulfates, ammonia and the like which will crack or dissolve the grain to reveal substantial internal image. Low concentration of excess halides are some times desirable in a developer or incorporated in the emulsion as halide releasing compounds but high concentrations are generally avoided to prevent substantial cracking of the grain, especially with respect to iodide releasing compounds.

Typical silver halide developing agents which can be used in the surface developers employed in the practice of the invention include hydroquinones, catechols, aminophenols, 3-pyrazolidones, ascorbic acid and its derviatives, reductones, phenylenediamines and the like or combinations of these. The developing agents can be incorporated in the photographic elements wherein they are brought into contact with the silver halide after imagewise exposure; however in certain embodiments they are preferably employed in the developing bath.

6 The surface developer compositions used in the process of the invention can also contain certain antifoggants and development restrainers or optionally they can be incorporated in layers of the photographic element. Typically useful antifoggants include nitrobenzoimidazoles, benzothiazoles, such as S-nitrobenzothiazole and 5-methylbenzothiazole, heterocyclic thiones such as l-methyl-2- tetraazaline-S-thione, aromatic and alaphatic mercapto compounds and the like.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A silver bromoiodide emulsion having an average silver halide grain size of about 1 micron is prepared by adding an aqueous solution of potassium bromide and an aqueous solution of silver nitrate, containing 0.75

mole percent potassium iodide, simultaneously to a rap is added to the emulsion at a concentration of 200 milligrams of dye per mole of silver. The spectrally sensitized emulsion is then coated at 400 milligrams of silver per ft. and 1,000 milligrams of gelatin per ft? on a film support.

Samples of the resulting photographic element are sensitometrically exposed to light and then subsequently exposed uniformly to an electronic flash filtered for a maximum Wavelength transmission of 380 nanometers. The images in the resulting coatings are then developed in an Elon-hydroquinone developer, i.e. Kodak D-l9, for 4 minutes at 20 C.

A direct positive image results having excellent clean out, i.e. a D in the exposed area of less than 0.1, and high maximum density, i.e. a maximum density greater than 1.0 in the unexposed image areas.

The oxidation potential of the spectral sensitizing dye of this example is plus 0.87 v. The reduction potential of this dye is minus 1.06 v.

The described dye has a region of spectral response of 500 to 680 nanometers and a relative reversal speed for the photographic element of 320.

EXAMPLES 2-5 The procedure set out in Example 1 is repeated with the exception that the dyes described in Examples 2-5 are individually employed in place of the described dye of Example 1. The dye structure, oxidation potential in volts, reduction potential in volts, region of spectral response and relative speed for the photographic elements employing these dyes is set out in following Tables I and II.

TABLE I Example 0. Dye structure Emanuel; (v.)

redu ntion (V.)

1 5,5'-diemote-3,3',9-triethylthlacarboeyanine bromide I Et Br- 2 Anhydro-S '-diohloro-3,9-dlethyl-3-(3-su1fobutyl)thlacarbocyanine hydroxide.

3 Anhydro-3,9-diethyl-5.5-dimethoxy-3-(3-sulfopropyl)thla:

carboeyanine hydroxide,

4 1-carboxymethyl-5-[(3-ethyl-2-benzoxazollnylidene ethylldenel- 3-phen(l)-2-th1ohydantoin.

5 Anhydro;3-ethyl-9-methyl-3-(4-su1fobutyl)thiacarbocyanlne +0.73

hydroxr e.

TABLE II Region of 5 5 spectral Relative response reversal (11111.) speed EXAMPLE 6 An internally sensitive silver chlorobromide gelatin emulsion is prepared using the halide conversion technique as described in Davey et al. U.S. Pat. 2,592,250. The emulsion is washed and the sensitizing dye as described in Example 1 is added at 200 milligrams per silver mole. The spectrally sensitized emulsion is then coated on a film support, image exposed, uniformly exposed to an electronic flash filtered for a maximum wavelength transmission of 380 nanometers and developed for 4 minutes at 20 C. in Kodak Developer D-19. A direct positive image having a discrimination greater than 1.0 is observed.

EXAMPLE 7 This is a comparative example. The procedure set out in Example 1 is repeated with the exception that a dye of the structure C=CH-C Et CI I E 0.95

7 is employed in place of the dye described in Example 1. Employing this dye and the procedure of Example 1 no reversal image is produced.

EXAMPLE 8 This is a comparative example.

The procedure set out in Example 1 is repeated with the exception that a dye of the structure is employed in place of the dye described in Example 1.

Employing the procedure of Example 1 with this dye no reversal image is produced.

Examples 7 and 8 illustrate that dyes having similar structures but which do not have the oxidation potential described as important according to the process of the invention, do not provide the results the process of the invention requires.

References disclosing the described dyes are U.S. application Ser. No. 56,702 of Gilman et al., filed July 20, 1970; British Pat. 1,128,840, published Oct. 2, 1968; Levkoev et al., J. of General Chemistry U.S.S.R., No. 16, vol. 9, pages 1189-94 (1946); U.S. Pat. 2,917,516 of Brooker et al., issued Dec. 15, 1959; U.S. Pat. 3,432,303 of Jones et al., issued Mar. 11, 1969; and U.S. Pat. 3,397,303 of Smith, issued Aug. 13, 1968.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

What is claimed is:

1. A process for producing a positive image in a silver halide element consisting essentially of 1) imagewise exposing a photographic element comprising a support having thereon at least one layer of a silver halide emulsion which comprises internal image silver halide grains which (a) are internally sensitive, (b) are free of intentional surface chemical sensitization and (e) have a sensitizing concentration of a hole trapping agent which is a methine or polymethine spectral sensitizing dye adsorbed to the surface of said grains, said dye having an anodic polarographic halfwave potential less positive than plus 0.90 volt and a cathodic polarographic halfwave potential more negative than minus 1.0 volt, said emulsion prior to the addition of hole trapping agent being characterized as one which when examined according to normal photographic testing techniques, by coating a test portion on a film support, with no image exposure and developing for about 5 minutes at 20 C. in Kodak Developer D-19 (surface developer), has a maximum density (fog) less than 0.25 and, when developing a separate coated sample for about 5 minutes at 20 C. in Kodak Developer D-19 containing 0.5 g. of potassium iodide per liter (internal developer), has a maximum density greater than 0.5, (2) exposing said exposed photographic element to an overall flash of light having an intensity which is at least 10 times the intensity of the imagewise exposure, and (3) developing a positive image in said photographic element with a surface, silver halide developer.

2. The process of claim 1 wherein said spectral sensitizing dye is anhydro-S,5'-dichloro-3,9-diethyl-3'-(3- sulfobutyl)thiacarbocyanine hydroxide.

3. The process of claim 1 wherein said spectral sensitizing dye is anhydro-3,9-diethyl-S,5'-dimethoxy-3'-(3- sulfopropyl)thiacarbocyanine hydroxide.

4. The process of claim 1 wherein said spectral sensitizing dye is anhydro-3-ethyl-9-methyl-3'-(4-sulfobutyl) thiacarbocyanine hydroxide.

5. The process of claim 1 wherein the concentration of hole trapping agent is from about 4.0 milligrams to about 400 milligrams per mole of silver halide.

References Cited UNITED STATES PATENTS 3,586,505 6/1971 Ridgway 96--107 2,497,876 2/ 1950 Fallesen et al. 96-64 3,537,858 ll/1970 Wise 96126 3,364,026 1/ 1968 Rees 96-64 3,206,313 9/1965 Porter et a1. 96108 NORMAN G. TORCHIN, Primary Examiner W. LOUIE, J R., Assistant Examiner U.S. Cl. X.R. 96-107, 108 

